Environment Editor, The Sydney Morning Herald

A burst of growth in the wake of drought-breaking rains across inland Australia in 2011 helped to turn the country into one of the world's biggest carbon sinks, new research has found.

While oceans and land typically absorb about half the carbon emissions caused by fossil-fuel burning and land clearing, the unusually strong La Nina weather event and associated heavy rains resulted in land vegetation alone accounting for 40 per cent of the CO2 take-up that year.

Australia contributed about 60 per cent of the additional worldwide carbon sink, revealing a larger role for semi-arid regions than previously known, the lead author of a paper published on Thursday in Nature journal, Benjamin Poulter, said.

''Semi-arid regions have been overlooked in the carbon cycle because they have low productivity and because they store low amounts of carbon in vegetation and soils in comparison with tropical or boreal systems,'' said Dr Poulter, now an assistant professor at Montana State University after leading the study at France's Laboratoire des Sciences du Climat et l'Environnement.

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The research also found Australia has been on a greening trend, with vegetation expanding 6 per cent since 1981. The carbon uptake of plants has apparently become four times as sensitive to rainfall.

However, the scientists wrote that while drier regions could rival rainforests in carbon take-up, the more transitory nature of the vegetation meant its role as a sink could quickly reverse.

''The carbon that was stored in vegetation in 2011 was relatively quickly returned back to the atmosphere because of annual mortality of semi-arid grasses or because of increased fire activity consuming the greater fuel loads,'' Professor Poulter said.

''Changes in the behaviour of dryland systems in the global carbon cycle are very short-lived, and contribute mainly to larger inter-annual atmospheric CO2 variability,'' he said.

An indication of how temporary those carbon gains are may be on display in the coming year. The Bureau of Meteorology confirmed on Tuesday that an El Nino event had at least a 70 per cent chance of forming this year.

El Ninos, the flip side of La Ninas, typically bring drier and hotter than usual conditions to eastern and southern Australia. Bushfire seasons also tend to be more eventful.

Professor Poulter said dryland vegetation appeared to be expanding in part because of climate change but also because of changes to fire management.

Increased atmospheric levels of C02 were also affecting the physiology of plants.

''Openings on the leaf surface, stomata, tend to decrease in either density or in size, and so plants are able to reduce water losses in enriched CO2 environments,'' Professor Poulter said.

Policy changes, such as land clearing laws in Queensland and NSW, also influence the amount of dryland vegetation growth, he said.